1. Field of the Invention
The present invention relates to a power output device.
2. Description of the Related Art
As a conventional power output device, Japanese Patent Application Laid-open Nos. Hei 10-337047 and 11-178114, for example, disclose a device comprising a capacitor connected to the positive and negative bus bars of an inverter circuit which supplies three phase AC power to a motor, and a DC power source connected to the positive or negative bus bar of the inverter circuit and to the neutral point of the motor. This device uses a circuit comprising the motor coils of the respective phases and the inverter switching elements of the respective phases as a voltage booster chopper circuit to boost the voltage of the DC power source to charge the capacitor using the boosted voltage, and drives the motor using the capacitor thus charged as a DC power source. Motor driving and capacitor charging are simultaneously controlled through switching of the inverter switching elements when three phase AC power is supplied to the motor.
This type of power output device may cause ripples in motor torque or a large copper loss. That is, in the device, potential at the motor neutral point is fixed relative to that of the positive or negative bus bar because the DC power source is connected to either the positive or negative bus bar of the inverter circuit and to the motor neutral point. The voltage between the capacitor terminals (an inter-terminal voltage), namely, a potential difference between the positive and negative bus bars of the inverter circuit, can be set higher than that of the DC power source because the voltage of the DC power source is boosted. When rectangular wave control is executed by supplying a rectangular wave having a frequency equal to the motor electrical angle to drive the motor, a time period when each phase of a rectangular-waved three phase power generated by the inverter circuit has a negative voltage relative to the potential at the neutral point and a time period when each phase has a positive voltage relative to the potential are equal to each other when the inter-terminal voltage of the capacitor is exactly twice as large as the voltage of the DC power source. However, when the inter-terminal voltage is not exactly twice as large as the voltage of the DC power source, these time periods are different. In this case, a DC component current may flow into the motor windings, causing ripples in motor torque. The above described power output device, which uses a capacitor as at least one of its power sources, can control the inter-terminal voltage, and thus avoid the above described problem. However, a power output device which uses a general DC power source instead of a capacitor can neither control the voltage of the DC power source nor solve the above problem due to the product irregularity and altered voltage of the DC power source due to its successive use.
It is accordingly an object of the present invention to provide a power output device which can avoid ripples in motor torque even when the potential at the motor neutral point differs from the average of the potentials at the positive and negative bus bars of the inverter circuit. It is another object of the present invention to provide a power output device capable of supplying a current in a desired direction to the motor neutral point.
In order to achieve, at least partially, the objects of the present invention, according to a first aspect of the present invention, there is provided a power output device, comprising
a motor designed to be driven to rotate using multiple phase AC power;
an inverter circuit supplying multiple phase AC power to the motor while switching a plurality of switching elements;
a first power source connected to a positive bus bar and a negative bus bar of the inverter circuit;
a second power source connected to the positive bus bar or the negative bus bar of the inverter circuit and to a neutral point of the motor; and
a controller for controlling switching of the plurality of switching elements of the inverter circuit such that a ratio between positive and negative voltages relative to a potential at the neutral point of the motor, concerning respective phases of the multiple phase AC power to be supplied to the motor, become a predetermined ratio based on a voltage of the first power source and a voltage of the second power source.
In this power output device, the controller controls switching of the plurality of switching elements of the inverter circuit such that a ratio between positive and negative voltages relative to a potential at the neutral point of the motor, concerning respective phases of the multiple phase AC power to be supplied to the motor, become a predetermined ratio based on a voltage of the first power source and a voltage of the second power source. In this control, the predetermined ratio is a ratio that would make a difference between a time integration value of a voltage of a positive pulse and an absolute value of a time integration value of a voltage of a negative value become a predetermined value. When the predetermined value is zero, control is carried out such that no current flows into the neutral point of the motor so that ripples in motor torque can be suppressed. When the predetermined value is any value other than zero, control is carried out such that a current in a predetermined direction flows into the neutral point of the motor.
In this power output device according to the first aspect of the invention, the first power source may be chargeable using power supplied from the second power source.
According to the second aspect of the present invention, there is provided a power output device, comprising:
a motor designed to be driven to rotate using multiple phase AC power;
an inverter circuit supplying multiple phase AC power to the motor while switching a plurality of switching elements;
a first power source connected to a positive bus bar of the inverter circuit and to a neutral point of the motor;
a second power source connected to the negative bus bar of the inverter circuit and to a neutral point of the motor; and
a controller for controlling switching of the plurality of switching elements of the inverter circuit such that a ratio between positive and negative voltages relative to a potential at the neutral point of the motor, concerning respective phases of the multiple phase AC power to be supplied to the motor, become a predetermined ratio based on a voltage of the first power source and a voltage of the second power source.
In this power output device according to a second aspect of the invention, the controller controls switching of the plurality of switching elements of the inverter circuit such that a ratio between positive and negative voltages relative to a potential at the neutral point of the motor, concerning respective phases of the multiple phase AC power to be supplied to the motor, become a predetermined ratio based on a voltage of the first power source and a voltage of the second power source. In this control, the predetermined ratio may be a ratio that would make a difference between a time integration value of a voltage of a positive pulse and an absolute value of a time integration value of a voltage of a negative value become a predetermined value. When the predetermined value is zero, control is carried out such that no current flows into the neutral point of the motor so that ripples in motor torque can be suppressed. When the predetermined value is any value other than zero, control is carried out such that a current in a predetermined direction flows into the neutral point of the motor.
In this power output device according to a second aspect of the present invention, the first power source may be chargeable using power supplied from the second power source, and the second power source may be chargeable using power supplied from the first power source.
A power output device according to the second aspect of the present invention may comprise a chargeable and dischargeable capacitor, connected to the positive bus bar and the negative bus bar of the inverter circuit.
In a power output device according to the first or second aspect of the present invention, which uses a ratio which would make a difference between a time integration value of a positive pulse and the absolute value of a time integration value of a negative pulse become a predetermined value, the controller adjusts a ratio between the positive pulse and the negative pulse, concerning the respective phases of the multiple phase AC power, such that the difference becomes the predetermined value.
Also, in a power output device according to the first or second aspect of the present invention, which uses a ratio which would make a difference between a time integration value of a positive pulse and the absolute value of a time integration value of a negative pulse become a predetermined value, the controller adjusts an effective voltage of either the positive pulse or the negative pulse, concerning the respective phases of the multiple phase AC power, such that the difference becomes a predetermined value. In this power output device, the effective voltage of the pulse is adjusted through pulse width modulation.
In a power output device according to the first or second aspect of the present invention, the motor may be a power generation motor capable of power generation in response to power input thereto. The first power source and/or the second power source may be a rechargeable and dischargeable power source. The power output device may comprise charge control for driving the motor so as to operate as a power generator, and controlling switching of the plurality of switching elements of the inverter circuit so as to charge the first power source and/or the second power source using power generated by the motor.